Lecture 16 Groundwater:

Size: px

Start display at page:

Download "Lecture 16 Groundwater:"

Pamela O’Connor’
5 years ago
Views:

1 Reading: Ch 6 Lecture 16 Groundwater: Today 1. Groundwater basics 2. inert tracers/dispersion 3. non-inert chemicals in the subsurface generic 4. non-inert chemicals in the subsurface inorganic ions Next time 1. Groundwater contamination, especially organic contaminants 2. Remediation methods Ground water = H 2 O that occupies pore spaces in the land's subsurface. The unsaturated (or vadose) zone is where most of the the weathering and soil forming processes we spoke of last week occur. This week we discuss chemicals in the saturated zone (below the water table, where the pore spaces are filled with water). The cycle of water movement at the land's surface is summarized below 1

lad surface and below ground: Sawhney and Brown. 1989.

2 Hydrologic zones and a monitoring well Organic chemical sources to groundwater and interactions on lad surface and below ground: Sawhney and Brown in 1993 text: 2

numerous contaminant sources to groundwater: we discussed some of the shallow ones last week Quick hydrologic primer Our treatment of the physical aspects of water flow will be very cursory.

3 numerous contaminant sources to groundwater: we discussed some of the shallow ones last week Quick hydrologic primer Our treatment of the physical aspects of water flow will be very cursory. Details of groundwater flow and contaminant hydrology are subjects of other courses. This quick summary covers chemistry-pertinent points: porosity: % pore volume relative to the rock volume. permeability: a quantitative measure of the ability of a substrate to pass fluid. Permeability does not have to be constant in all directions x, y and z in a substrate rock. permeability has a great deal to do with rock mineralogy and texture. Rock fracture is another control on isotropic the permeability field anisotropic Depending on the permeability field, groundwater (and their dissolved chemical constituents) will either move freely through the substrate or in a preferred direction. 3

4 Aquifer: saturated subsurface zone through which water flows. 4

5 Hydraulic conductivity is related to porosity and permeability, which over short length scales is related to grain size. Hydraulic conductivity Flow Regime - groundwater can migrate by: Diffuse flow (through large interconnected areas of ground substrate) Conduit flow (through a network of high permeability zones in otherwise low permeability rock). Chemical exchange between rock and water can be much less pronounced. 5

6 Hydraulic Head (h) drives water flow Pressure gradients within the subsurface cause groundwater to flow from regions of high to low pressure. Head (h) is measured in units of distance: h = z + p/ρ g + v 2 /2g z = elevation above arbitrary reference point; p = pressure; g = acceleration due to gravity; ρ = density; v = flow velocity Multiply through by ρg to determine water pressure: ρ gh = ρgz + p + ρ v 2 /2 z is gravitational potential energy related to vertical height - water will flow downhill converting potential E to kinetic E. p is the pressure term from overburden of water in a confined aquifer. v is a kinetic energy term, which is often neglected if water isn t moving very fast. In the context of the above terms we see that ρgh is an effective energy per unit volume that represents the sum of other terms and that h is an effective elevation. Darcy s law Ground water flows from areas of high head toward areas of low head (i.e., just as surface water flows down hill) q = -K dh/dx dimensionless head gradient term K is the hydraulic conductivity (units of velocity). q is the specific discharge in units of velocity (cm/s) but it easier to think of it as water volume flux across a unit area [(cm 3 /s)/cm 2 ] Does this look familiar? It has the same functional form as Fick s Law. 6

Inert chemicals in the subsurface: The rate of dispersion of groundwater contaminants and pollutants in the subsurface is largely controlled by the physical factors just discussed.

7 Inert chemicals in the subsurface: The rate of dispersion of groundwater contaminants and pollutants in the subsurface is largely controlled by the physical factors just discussed. Since the transport and fate of natural and pollutant chemicals in the subsurface are linked to physical aspects of flow, an initial site characterization study will examine solutes that can move at the rate of water flow (indicating little or no interaction with substrate surfaces). We call these solutes inert tracers. In these simple situations, the movement of a dissolved inert tracer is governed by physical effects related to water flow and substrate character only. A simple look at the effect of permeability on flow rate (as measured using a conservative (or inert ) tracer, i.e., one that, does not interact with substrate surfaces and moves at the rate of the water flow). 7

subsurface Dispersion causes a solute to be more spread out in space and time the farther

8 Different types of dispersion. Dispersion is a physical mechanism for spreading out solutes and for mixing in the subsurface Dispersion causes a solute to be more spread out in space and time the farther away one is from a concentrated source (e.g., a point source of contamination) in the subsurface... it occurs from a combination of eddy diffusion and advection 8

9 Remember the 1d diffusion-advection equation the relative roles of Advection (via Darcy's law) Dispersion (spreading) Adsorption (e.g., retardation) Decay/chemical reactions in subsurface solute transport Permeability effects on dispersion are strongest in nonisotropic media. Anisotropy may be the result of rock layering, bedding features, jointing, faulting... variations that occur at different length scales. Before attempting to understand the chemical properties of solute distribution in an aquifer a contaminant hydrologist or environmental geochemist would first determine the physical properties of a substrate in order to understand how water itself is moving through the subsurface. 9

For solutes that Do interact with the substrate chemical constituents can exchange between the water and the surrounding aquifer rock (also known as "substrate") during flow.

For instance, ion exchange reactions occurring during groundwater flow can cause the concentration of one element to decrease while another increases, as in the example below.

10 For solutes that Do interact with the substrate chemical constituents can exchange between the water and the surrounding aquifer rock (also known as "substrate") during flow. Ionic interactions cause many effects we are already familiar with. Recall our earlier discussions about the electric double layer, ZPC, ion exchange, CEC and ion sorption. For instance, ion exchange reactions occurring during groundwater flow can cause the concentration of one element to decrease while another increases, as in the example below. Unlike a charged surface on a colloidally suspended particle on a substrate solid the charged surfaces are fixed in place. The charge distribution clouds associated with one substrate surface will interact with those on a neighboring substrate surface. Surface charge causes anions and cations to flow through different parts of the subsurface pore-space network of a rock substrate (i.e., dissolved ions to move in a somewhat different manner than they would in the presence of free floating colloidal particles in surface water. 10

11 Recall that there is a ph dependency for net charge on most typical rock substrates and for metal speciation. ph, speciation, and surface charge combine to cause many heavy metals to bind or not bind over fairly narrow ranges of ph, making them either mobile or immobile in ground water systems. immobile mobile Retardation Ionic or lewis-acid-base interaction with the aquifer substrate causes these chemicals to move slower than the water itself (retardation). Retarded chamicals are still also subject to dispersive effects. Solute interaction can be described in most cases by Henry's Law of distribution (see also Chapter 5): [A] phase 1 = K eq [A] phase 2 or K eq = [A] phase 1 / [A] phase 2 (remember, this is the same form of the equation used to describe gas solubility in a fluid as a function of pressure) We use Henry's Law to describe solute interaction by notating K eq as "K d " (Henry's Law distribution coefficient) K d = [A] substrate / [A] water K >1 indicates that the solute "A" has a preference at equilibrium for interaction with the solid(s) in the system and it will therefore be retarded during flow. 11

12 The velocity of "A", ν A... through an aquifer is described by a retardation factor relative to the water flow itself, ν, which is 1 + (ρ/φ)k d, where ρ is the substrate grain density and φ is it's porosity. note: for K >1, ν A /ν <1 Retardation For a simple system of a homogeneous substrate composition, the distance a retarded solute (d A ) moves per unit time over the distance water moves over the same time interval is constant: (d A /d w ) 1 = (d A /d w ) 2 Let's examine the retardation in a sandstone aquifer of NH 4 + using the above equation. For K d = 2.43, ρ = 2.6 and φ = 0.28: ν NH4+ = ν H2O /[1+(2.6/0.28) 2.43] ν NH4+ = ν H2O. The ammonium ions will travel at only about 4% of ν H2O. 12

13 Retardation with other exchangeable ions. Retardation of NH 4+ in ground water of 7.4 meq/l going through the sandstone example above (where the substrate has 2.55 meq/100g CEC, and the ground water is at 1000 ppm NH 4+ ) ν NH4+ ~ 0.035ν H2O Notice that the pulse of NH 4 + ions disperses out as larger quantities of water are passed through the aquifer. Retardation also plays a key role in determining how organic contaminants move in groundwater systems. 1 pore vol. = volume of originally saturated pores The relative retardation of two elements We use an example of Sr and Cs (which are found in radioactive waste as the isotopes 90 Sr and 137 Cs) in a sandstone aquifer. For ρ = 1.7 and φ = 0.38 (characteristics of the sandstone) and K d-sr =10 and K d-cs = 100 (characteristics of the elements), we can calculate retardation of Sr and Cs of 45.7 and 448 (using the previously discussed equation) Relative velocities of Sr and Cs through this aquifer (independent of water flow rate): ν Sr = (ν H2O /retardation Sr ) = retardation Cs = 9.8 ν Cs (ν H2O /retardation Cs ) retardation Sr Typical aquifers have ρ/φ = 4 to 20, so retardation in general can have a big effect on solute transport for all but the lowest K d 13

14 Cation Saturated Substrates For solutes moving through a substrate whose cation exchange sites are already saturated (fully adsorbed) with solutes and/or for which there is more than one ion type in solution, we must return to our K d equation and consider ion competition. For two solutes A and B, [A + ] solid /[B + ] solid = K A-B [A + ] aq /[B + ] aq K A-B is the selectivity coefficient. Remember that we can quantify the amount of a chemical species in a system as a mole fraction "x i ". If we want to describe the amount of ion A as a proportion of the total exchangeable ions on a substrate surface, we use "x A-solids " (the mole fraction of A in the solids), where x A-solids = moles A-solids /moles Total ions-solids [A + ] solid /[B + ] solid = x A-solids /x B-solids = moles A-solids /moles Tot ions-solids moles B-solids /moles Tot ions-solids equating [A + ] solid /[B + ] solid from the two equations above gives us... K A-B [A + ] aq /[B + ] aq = moles A-solids /moles Tot ions-solids moles B-solids /moles Tot ions-solids We can further generalize this for "A" in competition with a bunch of ions "TOT", rather than just "B" [A + ] solid /([TOT]-[A + ]) solid = K AT [A + ] aq /([TOT]-[A + ]) aq note that [TOT] solid = CEC and [TOT] aq ~ TDS In a nutshell, this equation tells us that the amount of "A" sticking to a solid surface from aqueous solution is proportional to its: Henry's Law Distribution Constant the amount of "A" in solution relative to the TDS the CEC of the substrate. More complicated treatments of fluid flow and chemical retardation exist but are beyond the scope of this course. It is important to note that solutes are retarded by the aquifer substrate because of: dynamic equilibrium with the aqueous solution and the TDS makeup of the aqueous solution. Dispersion acts on top of these to distribute solutes of various retardations in space and time throughout the subsurface. 14

15 Relative retardation Cation breakthrough curves demonstrate combined effects of subsurface chemical reactions and dispersive effects on nonconservative solute movement. Relative concentrations (C/C o ) as a function of time after injection for each ion were measured 0.75 m away from the injection point of a salt solution. At C/C o = 1 the concentration is exactly the same as the injected concentration. Cl, typically a conservative ion, reaches a peak relative concentration of 1.0 at the monitor point after about 12 days, giving us an idea of the time scale for dispersion. Mg and K: C/C o < 1 at this time (Mg somewhat delayed in time). Na and Ca: peak C/C o > 1 (Ca reaches a value of 2, somewhat earlier than Cl - ). Mg and K were adsorbed by aquifer solids in exchange for Na and Ca. Selectivity coefficients indicate a preference for Ca over Mg, Na, K, but Ca was released to solution because of the concentrations of other cations were very elevated. Sorption and Saturation Effects A sharp down flow solute gradient is a related effect that is sometimes observed with continuous (as opposed to pulsed or one time) chemical streams (natural or contaminant). This is due to solute-particle adsorption until sorption sites are completely full. Continued fluid flow through a "saturated" domain will not exchange more of this solute (NH 4+, in this example), until fresh, unsaturated substrate is encountered. Once encountered, solute concentration drops dramatically, with a coincident increase in another solute (Ca 2+ in this example) released from the substrate by ion exchange. 15

16 Organic Solute Substrate retention Substrate retention reflects the affinity of an organic compound for ionic and Lewis acid/base interactions with surfaces. The more basic (electron donor) functional groups an organic molecule has, the more interaction it is likely to have with an inorganic substrate. There is a hierarchy of functional group types: Notice that carboxylic acids, alcohols, amines and thiols are at the high sorption side of the scale. Also notice that halogenated compounds are the least retentive of them all. The relative retardation of organic molecules can be determined in the lab using chromatographic columns of various substrate materials. This is the same approach used by analytical chemists to separate and purify both organic and inorganic materials out of a mixture. Substrate retention: quantified by examining the output with time of an aqueous solution of organic molecules passed through a substrate column. The retardation order for general classes of compounds is given in the table to the left. Many organic contaminants are "polyfunctional" and that these materials are usually highly retarded. The relative retardation amongst polyfunctional molecules follows the same "rules" as monofunctional molecules e.g., a dicarboxylic acid would probably be more retentive than a mono carboxylic acid-mono alcohol. Not listed are Quaternary amines (cations), which have very strong retention on clays (by analogy to NH 4+ ). Such amine compounds form a number of common herbicide classes. 16

17 Gas chromatograms are important tools for environmental geochemists. A chromatogram of 3 waters with more or less contamination (a "clean" tap water, a contaminated ground water, and a contaminated river water): Usually, a chromatogram is compared to a standard mixture of known contaminants. Contaminants in the unknown sample are identified by comparison with the standard chromatogram. Sometimes the output gas is injected into a mass spectrometer to aid in compound identification. A sample is heated a to dryness, vaporized, the resulting gas stream is sent through a substrate column that separates the components, which are detected on the end of the column in a variety of ways. Other chemical effects Organic rich plumes in the subsurface, such as landfill leachate, can also cause dramatic effects on Eh (or pe) in down flow groundwater. Recall that organisms use redox ladder reactions to decompose organic matter in these anoxic subsurface domains. Pe DOC NO 3-, Fe +3 and SO 4 2- reduction are the most common redox ladder reactions that occur here. Note that pe contours reach a minimum value (reducing conditions) at about the same location as the start of a maximum in DOC concentration. Such changes in DOC can cause other redox sensitive materials in the subsurface to change their mobility, which is particularly important for stored or spilled radioactive waste. 17

Chemical Hydrogeology

Physical hydrogeology: study of movement and occurrence of groundwater Chemical hydrogeology: study of chemical constituents in groundwater Chemical Hydrogeology Relevant courses General geochemistry [Donahoe]